Cell numbers are regulated by a balance among proliferation, growth arrest and programmed cell death (apoptosis). (Formace et al. 1992 Ann New York Acad. Sci. 663:139-153). Genes induced by various growth arrest and aptotic stimuli are the tumor suppressor gene p53, myeloid differentiation primary response genes (MyD genes), and growth arrest and DNA damage inducible genes (GADD genes) (Selvakumaran et al. 1994 Mol. Cell. Biol. 14:2352-2360).
Animal cells respond to differentiation signals which turn on or off the appropriate genes resulting in conversion of proliferating, undifferentiating cells into nonproliferating, highly specialized differentiated cells. An example of this process is the differentiation of myeloid precursor cells into mature granulocytes and macrophages. Blocks in the differentiation process appear to be a major step in tumor progression and lesions in genes involved in terminal differentiation contribute to the development of malignant tumors (Liebermann et al. 1994 Stem Cells 12:352-69).
Liebermann et al. suggest that MyD genes function as positive regulators of terminal hematopoietic cell differentiation, which is associated with inhibition of cell growth and apoptosis. Selvakumaran et al. supra provide evidence that MyD family member, murine MYD118, described as a terminal differentiation response gene, is expressed in M1D+ myeloid precursor cells following induction of terminal differentiation and growth arrest by IL6 and has been shown to be a positive regulator of apoptosis induced by TGFβ1. Additionally, leucine zipper transcription factors of the fos/jun family have been identified as MyD genes, specifically MyD21, MyD42 and MyD63, and function as positive regulators of hematopoietic cell differentiation, increasing the differentiation of myeloblastic leukemia cells in vitro and reducing the aggressiveness of the leukemic phenotype in nude mice. Liebermann et al. supra suggest that lesions in the MyD genes of the fos/jun family that affect expression or function of the genes contribute to development of leukemias.
The cDNA sequence and deduced amino acid sequence of murine MYD118 is disclosed in Abdollahi et al. (1991 Oncogene 6:165-167) who indicate that the cDNA nucleotide sequence of murine MyD118 predicts a protein of 160 amino acids, which does not contain protein secretory signals, transmembrane domains or known protein-DNA binding motifs, but does appear to contain a protein kinase phosphorylation site at position 204, two casein kinase II phosphorylation sites at positions 215 and 231 and several AT3 motifs in its 3′ untranslated region. Abdollahi et al. observed detectable levels of myd118 RNA in myeloid precursor enriched murine bone marrow, but not in several other non-myeloid murine tissues, such as liver or brain. Abdollahi et al. also observed that myd118 expression was induced in the absence of protein synthesis, following stimulation of M1D+ cells by IL-1, LPS and leukemia inhibitory factor (LIF).
The amino acid sequence for murine MYD118 is 75% similar (57% identical) to the amino acid sequence for murine GADD gene, GADD45, which is regulated in part by the tumor suppressor gene p53 (Zhan et al. 1994 Cancer Res. 54: 2755-60 and Carrier et al. J. Biol. Chem. 269:32672-32677). GADD and MYD118 are two separate but closely related genes and act synergistically to suppress growth of hematopoietic cell lineages.
Various portions of the nucleotide sequence encoding human MYD118 have been disclosed in the EST 1 and 2 database (version 92) of Genbank in cDNA libraries made from tissue from hippocampus (accession number M77995), retina (accession number H84533 and H83991), olfactory epithelium (accession number H71592), human fetal lung (accession number D310470 and D31559), breast (accession number H44355, R55161 and R82994), placenta (accession number R24009, R63425, R21918 and R22497), human white blood cells (accession number T33963), human brain (accession number T35368), human pancreas (accession number T29941), liver (accession number T40088), prostate gland (accession number T35225) and lung (accession number T35563). The complete nucleotide sequence encoding human MYD118 has not been disclosed.
Myeloproliferative diseases and leukemias are neoplasms of the hematopoietic stem cell and include acute lymphocytic leukemia (ALL), acute myelogenous leukemia (AML), chronic lymphocytic leukemia (CLL), hairy cell leukemia and chronic myelogenous leukemia (CML); polycythemia vera (PV); agnogenic myeloid metaplasia with myelofibrosis (AMM/MF); and essential thrombocytosis (ET). Research suggests that the myeloproliferative diseases arise as clonal expansions of a single transformed stem cell and that all of the myeloid cells of the blood are derived from the neoplastic clone. In leukemia, leukemic cells proliferate primarily in the bone marrow and lymphoid tissues and are characterized according to the cell type involved (myeloid or lymphoid). Acute leukemia is characterized by proliferation of immature myeloid or lymphoid cells. CLL is a hematologic neoplasm characterized by the accumulation of mature-appearing lymphocytes in the peripheral blood associated with infiltration of the bone marrow. Hairy cell leukemia is characterized by peripheral blood cytopenias, splenomegaly and malignant cells in the blood and bone marrow. CML is characterized by marked splenomegaly and the production of increased numbers of granulocytes, particularly neutrophils, in the marrow and blood. PV is characterized by splenomegaly and an increased production of all myeloid elements, but is dominated by an elevated hemoglobin concentration. AMM/MF is characterized by the tendency of the neoplastic stem cells to lodge and grow in multiple sites outside the marrow, progressive splenomegaly, the gradual replacement of marrow elements by fibrosis, and variable changes in the number of granulocytes and platelets. ET is characterized by an elevated platelet count and represents the overproduction of platelets in the absence of a recognizable stimulus. In cultures of bone marrow cells from individuals subject to ET, colonies of megakaryocytes from megakaryocyte progenitors form in the absence of added stimulus, whereas such colonies do not occur with marrow cell cultures from normal individuals. (Harrison's Principles of Internal Medicine, 1987 11th edition, Braunwald et al. Editors, McGraw-Hill Book Company, New York City).